The use of lithium titanate Li4Ti5O12, or lithium titanium spinel for short, in particular as substitute for graphite as anode material in rechargeable lithium-ion batteries was proposed some time ago.
A current overview of anode materials in such batteries can be found e.g. in: Bruce et al., Angew. Chem. Int. Ed. 2008, 47, 2930-2946.
The advantages of Li4Ti5O12 compared with graphite are in particular its better cycle stability, its better thermal load capacity as well as the higher operational reliability. Li4Ti5O12 has a relatively constant potential difference of 1.55 V compared with lithium and achieves several 1000 charge and discharge cycles with a loss of capacity of <20%.
Thus lithium titanate has a clearly more positive potential than graphite which has previously customarily been used as anode in rechargeable lithium-ion batteries.
However, the higher potential also results in a lower voltage difference. Together with a reduced capacity of 175 mAh/g compared with 372 mAh/g (theoretical value) of graphite, this leads to a clearly lower energy density compared with lithium-ion batteries with graphite anodes.
However, Li4Ti5O12 has a long life and is non-toxic and is therefore also not to be classified as posing a threat to the environment.
Recently, LiFePO4 has been used as cathode material in lithium-ion batteries, with the result that a voltage difference of 2 V can be achieved in a combination of Li4Ti5O12 and LiFePO4.
Various aspects of the production of lithium titanate Li4Ti5O12 are described in detail. Usually, Li4Ti5O12 is obtained by means of a solid-state reaction between a titanium compound, typically TiO2, and a lithium compound, typically Li2CO3, at high temperatures of over 750° C., as described in U.S. Pat. No. 5,545,468 or EP 1 057 783 A1.
Sol-gel methods for the production of Li4Ti5O12 are also described (DE 103 19 464 A1). Furthermore, production methods by means of flame spray pyrolysis are proposed (Ernst, F. O. et al. Materials Chemistry and Physics 2007, 101(2-3, pp. 372-378) as well as so-called “hydrothermal methods” in anhydrous media (Kalbac, M. et al., Journal of Solid State Electrochemistry 2003, 8(1) pp. 2-6).
Since the lithium titanate as electrode is typically compressed to an electrode with carbon, in particular graphite or carbon black, EP 1 796 189 A2 proposes providing complex lithium transition metal oxides ex situ, i.e. after their complete synthesis with a carbon-containing coating. A disadvantage with this method, however, is the large particle size of the contained product, in particular the secondary particle size. Moreover, the carbon coating in this method is located on the secondary and not the primary particles, which leads to poor electrochemical properties, in particular as regards its capacity behaviour.
There was therefore a need to provide a further lithium titanium oxide, in particular a lithium titanate Li4Ti5O12, which has particularly small particles and improved electrochemical properties.
According to the invention, this object is achieved by a carbon-containing lithium titanium oxide containing spherical (secondary) particle aggregates with a diameter of 1-80 μm consisting of lithium titanium oxide primary particles coated with carbon.
The German terms “Partikel” and “Teilchen” here are used synonymously to mean particle.
In the following, by lithium titanium oxide is meant a lithium titanate which according to the invention includes all lithium titanium spinels of the type Li1+xTi2−xO4 with 0≦x≦⅓ of the spatial group Fd3m and generally also all mixed lithium titanium oxides of the generic formula LixTiyO (0<x, y<1).
The carbon-coated lithium titanium oxide according to the invention consists, as mentioned, of secondary particles which are formed of primary particles coated with carbon. The secondary particles are spherical in shape.
The result of the particle size according to the invention of the secondary particles is that the current density in an electrode that contains the carbon-coated lithium titanium oxide material according to the invention is particularly high and it has a high cycle stability compared with the materials of the state of the art, in particular EP 1796 189 A2.
Surprisingly, it was also found that the carbon-containing lithium titanium oxide according to the invention has a BET surface area (measured in accordance with DIN 66134) of 1-10 m2/g, preferably <10 m2/g, still more preferably <8 m2/g and quite particularly preferably <5 m2/g. In a quite particularly preferred embodiment, typical values lie in the range of from 3-5 m2/g.
The primary particles coated with carbon typically have a size of <1 μm. It is important according to the invention that the primary particles are small and at least partially coated with carbon, with the result that the current-carrying capacity and the cycle stability of an electrode containing the lithium titanium oxide according to the invention are particularly high compared with non-carbon-coated materials or materials which are not homogeneously coated or compared with materials in which only the secondary particles are coated.
In preferred embodiments of the present invention, the carbon content of the lithium titanium oxide according to the invention is 0.05 to 2 wt.-%, in quite particularly preferred embodiments 0.05 to 0.5 wt.-%.
Surprisingly, it was found that relatively low carbon contents, i.e. thus a relatively thin carbon coating of the primary particles, are sufficient to bring about the above-mentioned advantageous effects in electrodes which contain the material according to the invention.
Of the lithium titanium oxides, Li4Ti5O12 is preferred because it is particularly well-suited as electrode material.